Fuel cell system

ABSTRACT

A fuel cell system that generates electric power by supplying anode gas and cathode gas to a fuel cell includes a control valve adapted to control the pressure of the anode gas to be supplied to the fuel cell; a buffer unit adapted to store the anode-off gas to be discharged from the fuel cell; a pulsation operation unit adapted to control the control valve in order to periodically increase and decrease the pressure of the anode gas at a specific width of the pulsation; and a pulsation width correcting unit adapted to correct the width of the pulsation on the basis of the temperature of the buffer unit.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No.14/122,790, filed Nov. 27, 2013, which is a National Stage Applicationof PCT/JP2012/060350, filed on Apr. 17, 2012, which claims benefit ofpriority from the prior Japanese Application No. 2011-124220, filed onJun. 2, 2011; the entire contents of all of which are incorporatedherein by reference.

TECHNICAL FIELD

The present invention relates to a fuel cell system.

BACKGROUND ART

The fuel cell system described in JP2007-517369A includes anormally-closed solenoid-operated valve in an anode gas supply passageand a normally-opened solenoid-operated valve and a buffer tank (arecycle tank) in series from upstream in an anode gas discharge passage.

This conventional fuel cell system is an anode gas non-recycling fuelcell system which does not return unused anode gas discharged to theanode gas discharge passage to the anode gas supply passage, carryingout the pulsation operation to increase or decrease a pressure of theanode gas with a width of the pulsation in accordance with theoperational status by means of a control valve for controlling the anodepressure.

SUMMARY OF INVENTION

However, the above-described conventional fuel cell system has not takeninto account the temperature change of the buffer tank. Therefore, theconventional fuel cell system is problematic in that stability ofelectric power generation is lowered by a low level of the anode gas inthe interior of a fuel cell stack or a discharge performance of liquidwater is deteriorated depending on the temperature of the buffer tankwhen the pulsation operation is carried out at the set width of thepulsation.

The present invention has been created in the light of theabove-described problems, with the object of carrying out stableelectric power generation and ensuring the discharge performance ofliquid water by setting an appropriate width of the pulsation inaccordance with the temperature of the buffer tank.

In order to attain the above-described object, a specific aspect of thepresent invention provides a fuel cell system comprising: a controlvalve adapted to control the pressure of the anode gas to be supplied tothe fuel cell; a buffer unit adapted to store the anode-off gas to bedischarged from the fuel cell; a pulsation operation means adapted tocontrol the control valve in order to periodically increase and decreasethe pressure of the anode gas at a specific width of the pulsation; anda pulsation width correcting means adapted to correct the width of thepulsation on the basis of the temperature of the buffer unit.

The embodiments and advantages of the present invention will behereinafter described in detail with reference to the attached drawings.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1A is a schematic perspective view of a fuel cell.

FIG. 1B is a sectional view of the fuel cell.

FIG. 2 is a schematic block diagram of an anode gas non-recycling fuelcell system according to a first embodiment of the present invention.

FIG. 3 is a view explaining the pulsation operation at a steadyoperation.

FIG. 4 is a view explaining a reason why a part of which anode gas levelis lower than those of other parts is locally generated in the interiorof an anode gas flow passage.

FIG. 5 is a view illustrating the lowest anode gas level in the flowpassage at the same width of the pulsation upon reduction of an anodepressure in accordance with the temperature of a buffer tank.

FIG. 6 is a view illustrating the intensity of kinetic energy of theanode gas at the same width of the pulsation upon increase of an anodepressure in accordance with the temperature of a buffer tank.

FIG. 7 is a flowchart explaining the control of the pulsation operationaccording to the first embodiment of the present invention.

FIG. 8 is a table for computing a reference pressure from the outputelectric current.

FIG. 9 is a table for computing a basic width of the pulsation from theoutput electric current.

FIG. 10 is a table for computing a basic aperture of a purge valve fromthe temperature of a fuel cell stack.

FIG. 11 is a flowchart explaining the low temperature pulsation widthcorrection processing according to the first embodiment of the presentinvention.

FIG. 12 is a map representing a relation between the width of thepulsation and the lowest anode gas level in the flow passage uponreduction of the anode pressure for each temperature of the buffer tank.

FIG. 13 is a flowchart explaining the high temperature pulsation widthcorrection processing according to the first embodiment of the presentinvention.

FIG. 14 is a table for computing the allowable lower limit kineticenergy from the output electric current.

FIG. 15 is a map representing a relation between the width of thepulsation and the kinetic energy of the anode gas upon increase of theanode pressure for each temperature of the buffer tank.

FIG. 16 is a view explaining the action of the low temperature pulsationwidth correction processing according to the first embodiment of thepresent invention.

FIG. 17 is a view explaining the action of the high temperaturepulsation width correction processing according to the first embodimentof the present invention.

FIG. 18 is a view illustrating a relation between a width of thepulsation ΔP and the lowest anode gas level in the flow passage when thetemperature of the buffer tank is a specific temperature in accordancewith the aperture of the purge valve.

FIG. 19 is a flowchart explaining the low temperature pulsation widthcorrection processing according to a second embodiment of the presentinvention.

FIG. 20 is a table for computing the correction amount of the apertureof the purge valve from the internal resistance.

FIG. 21 is a table for computing the correction amount of the allowablemaximum width of the pulsation from the correction amount of theaperture of the purge valve.

FIG. 22 is a view explaining the action of the low temperature pulsationwidth correction processing according to the second embodiment of thepresent invention.

FIG. 23 is a view illustrating a relation between the width of thepulsation, the kinetic energy of the anode gas, and the lowest anode gaslevel in the flow passage when the temperature of the buffer tank is aspecific temperature higher than the steady temperature of the fuel cellstack.

FIG. 24 is a flowchart explaining the high temperature pulsation widthcorrection processing according to a third embodiment of the presentinvention.

FIG. 25 is a table for computing the correction amount of the apertureof the purge valve from the difference in level.

FIG. 26 is a table for computing the correction amount of the apertureof the purge valve from the difference in level.

FIG. 27 is a view explaining the action of the high temperaturepulsation width correction processing according to the third embodimentof the present invention.

FIG. 28 is a view explaining the action of the high temperaturepulsation width correction processing according to the third embodimentof the present invention.

FIG. 29 is a flowchart explaining the control of the pulsation operationaccording to a fourth embodiment of the present invention.

FIG. 30 is a flowchart explaining the low temperature correctionprocessing of the aperture of the purge valve according to the fourthembodiment of the present invention.

FIG. 31 is a table for computing the correction amount of the apertureof the purge valve from the difference in level.

FIG. 32 is a view explaining the action of the low temperaturecorrection processing of the aperture of the purge valve according tothe fourth embodiment of the present invention.

DESCRIPTION OF EMBODIMENTS First Embodiment

A fuel cell formed by interposing an electrolyte membrane between ananode (a fuel electrode) and a cathode (an oxidizing agent electrode)generates electric power by supplying anode gas (fuel gas) containinghydrogen to the anode and cathode gas (oxidizing agent gas) containingoxygen to the cathode. Electrode reaction progressing in bothelectrodes, namely, the anode and the cathode is as follows:Anode:2H₂→4H⁺+4e ⁻  (1)Cathode:4H⁺+4e ⁻+O₂→2H₂O  (2)

These electric reactions (1) and (2) allow the fuel cell to generate anelectromotive force of about one volt.

FIG. 1A is a schematic perspective view of a fuel cell 10. FIG. 1B is aB-B sectional view of the fuel cell 10 of FIG. 1A.

The fuel cell 10 is configured by arranging an anode separator 12 and acathode separator 13 on both sides of a membrane electrode assembly(hereinafter, referred to as “MEA”) 11.

The MEA 11 is provided with an electrolyte membrane 111, an anode 112,and a cathode 113. The MEA 11 has the anode 112 on one side of theelectrolyte membrane 111 and the cathode 113 on the other side thereof.

The electrolyte membrane 111 is a proton-conducting ion-exchangemembrane made by a fluorinated resin. The electrolyte membrane 111 showsan excellent electric conductivity in a wet condition.

The anode 112 is provided with a catalyst layer 112 a and a gasdiffusion layer 112 b. The catalyst layer 112 a comes in contact withthe electrolyte membrane 111. The catalyst layer 112 a is made ofplatinum-supported carbon black particles or platinum, etc.-supportedcarbon black particles. The gas diffusion layer 112 b is located outsideof the catalyst layer 112 a (the other side of the electrolyte 111),coming in contact with the anode separator 12. The gas diffusion layer112 b is formed by a member having a sufficient gas diffuseness and aconductive property, for example, a carbon cloth woven by yearns made ofa carbon fiber.

The cathode 113 is also provided with a catalyst layer 113 a and a gasdiffusion layer 113 b along with the anode 112.

The anode separator 12 comes in contact with the gas diffusion layer 112b. The anode separator 12 has a plurality of groove-like anode gas flowpassages 121 for supplying anode gas to the anode 112 on the side comingin contact with the gas diffusion layer 112 b.

The cathode separator 13 comes in contact with the gas diffusion layer113 b. The cathode separator 13 has a plurality of groove-like cathodegas flow passages 131 for supplying cathode gas to the cathode 113 onthe side coming in contact with the gas diffusion layer 113 b.

The anode gas flowing through the anode gas flow passages 121 and thecathode gas flowing through the cathode gas flow passages 131 flow inparallel to each other in the same direction. They may flow in parallelto each other in the reverse direction.

In the case of using such a fuel cell 10 as an automotive power source,a fuel cell stack with hundreds of fuel cells 10 piled up is used sincelarge electric power is required. Then, electric power for driving avehicle is taken by configuring a fuel cell system for supplying anodegas and cathode gas to the fuel cell stack.

FIG. 2 is a schematic block diagram of an anode gas non-recycling fuelcell system 1 according to the embodiment of the present invention.

The fuel cell system 1 is provided with a fuel cell stack 2, an anodegas supplier 3, and a controller 4.

The fuel cell stack 2 formed by piling up a plurality of fuel cells 10generates electric power by being supplied with anode gas and cathodegas, generating electric power required for driving a vehicle (forexample, electric power required for driving a motor).

Illustrations of a cathode gas supplier and discharger for supplying anddischarging cathode gas to the fuel cell stack 2 and a cooling systemfor cooling the fuel cell stack 2 are omitted in order to facilitateunderstanding of the present invention since they are not main parts ofthe present invention. The present embodiment uses air as cathode gas.

The anode gas supplier 3 is provided with a high pressure tank 31, ananode gas supply passage 32, a pressure-adjusting valve 33, a pressuresensor 34, an anode gas discharge passage 35, a buffer tank 36, a purgepassage 37, and a purge valve 38.

The high pressure tank 31 stores the anode gas to be supplied to thefuel cell stack 2 while keeping it at a high pressure.

The anode gas supply passage 32 is a passage for supplying the anode gasdischarged from the high pressure tank 31 to the fuel cell stack 2,wherein one end of which is connected to the high pressure tank 31 andthe other end of which is connected to an anode gas inlet hole 21 of thefuel cell stack 2.

The pressure-adjusting valve 33 is located in the anode gas supplypassage 32. The pressure-adjusting valve 33 adjusts the anode gasdischarged from the high pressure tank 31 to the desired pressure, then,supplies it to the fuel cell stack 2. The pressure-adjusting valve 33 isan electromagnetic valve capable of continuously or step-by-stepadjusting the aperture that is controlled by the controller 4.

The pressure sensor 34 is located in the anode gas supply passage 32located downstream of the pressure-adjusting valve 33. The pressuresensor 34 detects the pressure of the anode gas flowing through theanode gas supply passage 32 located downstream of the pressure-adjustingvalve 33. According to the present embodiment, the pressure of the anodegas detected by this pressure sensor 34 is used as the pressure of theentire anode system including respective anode gas flow passages 121 inthe interior of the fuel cell stack and the buffer tank 36 (hereinafter,referred to as “an anode pressure”).

The anode gas discharge passage 35 has the end connected to an anode gasoutlet hole 22 of the fuel cell stack 2 and the other end connected tothe top of the buffer tank 36. The mixture gas of the redundant anodegas that has not been used for electrode reactions and the impure gassuch as nitrogen and moisture that has transmitted from the cathode tothe anode gas flow passage 121 (hereinafter, referred to as “anode-offgas”) is discharged to the anode gas discharge passage 35.

The buffer tank 36 once stores the anode-off gas that has flown throughthe anode gas discharge passage 35. Moisture in the anode-off gaspartially becomes liquid water by being condensed in the buffer tank 36,being separated from the anode-off gas.

The purge passage 37 has the end connected to the bottom of the buffertank 36. The other end of the purge passage 37 is an opening end. Theanode-off gas and liquid water stored in the buffer tank 36 aredischarged from the opening end to external air through the purgepassage 37. They are normally discharged to a cathode discharge linealthough not illustrated in FIG. 2.

The purge valve 38 is located in the purge passage 37. The purge valve38 is an electromagnetic valve capable of continuously or step-by-stepadjusting the aperture that is controlled by the controller 4. Theamount of anode-off gas to be discharged from the buffer tank 36 toexternal air via the purge passage 37 is adjusted through adjustment ofthe aperture of the purge valve 38, further, the anode gas level in thebuffer tank 36 is adjusted such that it becomes a constant level. If theoperational status of the fuel cell system 1 is identical, the more theaperture of the purge valve 38 is increased, the more the nitrogen levelin the buffer tank 36 is decreased and the more the anode gas level isincreased.

The controller 4 is configured by a microcontroller including a centralprocessing unit (CPU), a random access memory (RAM), and an input/outputinterface (I/O interface).

The controller 4 receives signals for detecting the operational statusof the fuel cell system 1 from a sensor, other than the above-descriedpressure sensor 34, such as an electric current sensor 41 for detectingthe output current of the fuel cell stack 2, a temperature sensor 42 fordetecting a temperature of cooling water that cools the fuel cell stack2 (hereinafter, referred to as “a stack temperature”), and anaccelerator stroke sensor 43 for detecting the depression amount of theaccelerator pedal (hereinafter, referred to as “accelerator operationalamount”).

The controller 4 carries out the pulsation operation for periodicallyincreasing and decreasing the anode pressure by periodically opening andclosing the pressure-adjusting valve 33 on the basis of these inputsignals and maintains the anode gas level in the buffer tank 36 at aspecific level by adjusting the flowing amount of the anode gas to bedischarged from the buffer tank 36 through adjustment of the purge valve38.

In the case of the anode gas non-recycling fuel cell system 1, if thefuel cell system 1 continuously supplies the anode gas from the highpressure tank 31 to the fuel cell stack 2 while leaving thepressure-adjusting valve 33 open, the anode-off gas containing theunused anode gas discharged from the fuel cell stack 2 wastefullycontinues to be discharged from the buffer tank 36 to external air viathe purge passage 37.

Therefore, according to the present embodiment, the pulsation operationfor periodically increasing and decreasing the anode pressure is carriedout by periodically opening and closing the pressure-adjusting valve 33.It is possible to allow the anode-off gas stored in the buffer tank 36to flow backward to the fuel cell stack 2 upon reduction of the anodepressure by carrying out the pulsation operation. Thereby, it ispossible to decrease the anode gas amount to be discharged to externalair since the anode gas in the anode-off gas can be reused, and as aresult, it can be more efficient.

Hereinafter, the pulsation operation, as well as the reason why theanode-off gas stored in the buffer tank 36 flows backward to the fuelcell stack 2 upon reduction of the anode pressure, will be explainedwith reference to FIG. 3.

FIG. 3 is a view explaining the pulsation operation when the operationstatus of the fuel cell system 1 is a steady operation.

As illustrated in FIG. 3(A), the controller 4 computes the referencepressure and the width of the pulsation of the anode pressure on thebasis of the load applied to the fuel cell stack 2 (hereinafter,referred to as “a stack load”) (output current), setting the upper limitand the lower limit of the anode pressure. Then, the anode pressure isperiodically increased and decreased between the upper limit and thelower limit of the set anode pressure by periodically increasing anddecreasing the anode pressure within the range of the width of thepulsation on the basis of the reference pressure.

Specifically, when the anode pressure reaches the lower limit at a timet1, as illustrated in FIG. 3(B), the pressure-adjusting valve 33 isopened to at least the aperture at which the anode pressure can beincreased to the upper limit. In this state, the anode gas is dischargedto the buffer tank 36 after being supplied from the high pressure tank31 to the fuel cell stack 2.

If the anode pressure reaches the upper limit at a time t2, asillustrated in FIG. 3(B), the pressure-adjusting valve 33 is fullyopened, stopping supply of anode gas from the high pressure tank 31 tothe fuel cell stack 2. Then, since the anode gas left in the anode gasflow passage 121 in the interior of the fuel cell stack is consumed astime advances in accordance with the above-described electrode reaction(1), the anode pressure is reduced for the consumed amount of the anodegas.

In addition, if the anode gas left in the anode gas flow passage 121 isconsumed, the anode-off gas flows backward from the buffer tank 36 tothe anode gas flow passage 121 since the pressure of the buffer tank 36becomes temporarily higher than the pressure of the anode gas flowpassage 121. As a result, the anode gas left in the anode gas flowpassage 121 and the anode gas in the anode-off gas flowed backward tothe anode gas flow passage 121 are consumed as time advances and theanode pressure is further lowered.

When the anode pressure reaches the lower limit at a time t3, thepressure-adjusting valve 33 is opened along with the case at the timet1. Then, if the anode pressure reaches the upper limit again at a timet4, the pressure-adjusting valve 33 is fully closed.

Here, the above-described reference pressure and width of the pulsationof the anode pressure are set on the basis of the premise that thetemperature of the fuel cell stack 2 is identical to the temperature ofthe buffer tank 36. Specifically, they are set on the basis of thepremise that the temperature of the buffer tank 36 is identical to thesteady temperature of the fuel cell stack 2 when warming up of the fuelcell stack 2 is completed (about 60 C°).

However, the temperature of the buffer tank 36 is sometimes lower thanthe steady temperature of the fuel cell stack 2 during warming up of thefuel cell stack 2. In addition, the temperature of the buffer tank 36 issometimes lower or higher than the steady temperature of the fuel cellstack 2 since it is varied even after completion of worming up of thefuel cell stack 2 in accordance with the external environments such asan ambient temperature and an air resistance.

It has been found that the following respective problems are caused uponreduction of the anode pressure from the upper limit pressure to thelower limit pressure and upon increase of the anode pressure from thelower limit pressure to the upper limit pressure if the width of thepulsation is set without taking into account such change in temperatureof the buffer tank 36 in the case of carrying out the pulsationoperation.

First, the problem caused upon reduction of the anode pressure will bedescribed.

The amount of material in the anode gas (hydrogen) located in theinterior of the buffer tank 36 when the anode pressure reaches aspecific upper limit pressure is varied in accordance with thetemperature of the buffer tank 36. Specifically, if the pressure in theinterior of the buffer tank 36 is the same, the lower the temperature ofthe buffer tank 36 becomes, the more the amount of material in the anodegas located in the interior of the buffer tank 36 is increased.

The anode pressure is reduced from the upper limit pressure to the lowerlimit pressure upon reduction of the anode pressure by consuming theanode gas left in the anode gas flow passage 121 and the anode gas inthe interior of the anode-off gas flowed backward to the anode gas flowpassage 121. Therefore, the time required for lowering the anodepressure to the lower limit becomes long since the more the amount ofmaterial in the anode gas located in the interior of the buffer tank 36is increased, the more the consumed amount of the anode gas needed forlowering the anode pressure to the lower limit is increased.

Here, a part of which anode gas level is lower than the levels of otherparts is locally generated within the anode gas flow passage 121 uponreduction of the anode pressure. The reason for this will be describedwith reference to FIG. 4.

FIG. 4 is a view explaining a reason why a part of which anode gas levelis lower than those of other parts is locally generated within the anodegas flow passage 121. FIG. 4(A) is a view illustrating flows of theanode gas and the anode-off gas in the interior of the anode gas flowpassage 121 upon reduction of the anode pressure. FIG. 4(B) is a viewillustrating the level distribution of the anode gas in the interior ofthe anode gas flow passage 121 upon reduction of the anode pressure astime advances.

As illustrated in FIG. 4(A), the anode-off gas flows backward from thebuffer tank 36 side to the anode gas flow passage 121 since the pressureof the buffer tank 36 is temporarily higher than that of the anode gasflow passage 121 if the anode gas left in the anode gas flow passage 121is consumed. In addition, the high level of anode gas located in theanode gas supply passage 32 similarly flows into the anode gas flowpassage 121 of which pressure is low.

Then, a stagnation point where each gas flow rate becomes substantiallyzero is generated at the injunction point of the anode gas flowing fromthe anode gas supply passage 32 side into the anode gas flow passage 121and the anode-off gas that has flown backward from the buffer tank 36side to the anode gas flow passage 121.

If such a stagnation point is generated within the anode gas flowpassage 121, nitrogen in the interior of the anode-off gas that is notused for the above-described electrode reactions (1) is stored in thevicinity of the stagnation point as time advances. As a result, thenitrogen level in the vicinity of the stagnation point is higher thanthe levels of other parts, and the anode gas level in the vicinity ofthe stagnation point is decreased less than the levels of other parts astime advances as illustrated in FIG. 4(B).

If the anode gas level in the part where the anode gas level is lowestwithin the anode gas flow passage 121 (hereinafter, referred to as “thelowest anode gas level in the flow passage”) is lower than thepredetermined allowable lower limit anode gas level, the voltage ispossibly turned into a negative voltage since the above-describedelectrode reactions (1) and (2) are prevented and this causes thedeterioration of the fuel cell 10.

Accordingly, it has to avoid the lowest anode gas level in the flowpassage from falling below the allowable lower limit anode gas levelupon reduction of the anode pressure.

However, if the pulsation operation is carried out at the width of thepulsation that is set on the basis of the premise that the temperatureof the buffer tank 36 is the steady temperature of the fuel cell stack 2when the temperature of the buffer tank 36 is lower than the steadytemperature of the fuel cell stack 2, the time required for lowering theanode pressure to the lower limit pressure becomes long, possiblycausing the lowest anode gas level in the flow passage to fall below theallowable lower limit anode gas level.

Therefore, according to the present embodiment, when the temperature ofthe buffer tank 36 is low, the time required for lowering the anodepressure to the lower limit pressure is shortened by correcting thewidth of the pulsation to be small. Thereby, the lowest anode gas levelin the flow passage is prevented from falling below the allowable lowerlimit anode gas level.

FIG. 5 is a view illustrating the lowest anode gas level in the flowpassage at the same width of the pulsation upon reduction of the anodepressure in accordance with the temperature of the buffer tank 36.

As illustrated in FIG. 5, if the width of the pulsation is the same, ithas been found that the lower the temperature of the buffer tank 36becomes, the longer the time required for lowering the anode pressure tothe lower limit pressure becomes, and the lowest anode gas level in theflow passage is lowered.

Subsequently, the problem caused upon increase of the anode pressurewill be described.

As described above, the amount of material in the anode gas (hydrogen)located in the interior of the buffer tank 36 when the anode pressurereaches a specific upper limit pressure is varied in accordance with thetemperature of the buffer tank 36. Specifically, if the pressure in theinterior of the buffer tank 36 is the same, the higher the temperatureof the buffer tank 36 becomes, the more the amount of material in theanode gas located in the interior of the buffer tank 36 is decreased.

Therefore, the higher the temperature of the buffer tank 36 becomes, themore the amount of the anode gas required to increase the anode gas tothe upper limit pressure is reduced. As a result, the higher thetemperature of the buffer tank 36 becomes, the more the flow rate of theanode gas upon increase of the anode pressure and therefore the kineticenergy are reduced, then the discharge performance of liquid water inthe interior of the anode gas flow passage 121 is deteriorated.

Accordingly, if the pulsation operation is carried out at the width ofthe pulsation that is set on the basis of the premise that thetemperature of the buffer tank 36 is identical to the steady temperatureof the fuel cell stack 2 when the temperature of the buffer tank 36 ishigher than the steady temperature of the fuel cell stack 2, the kineticenergy of the anode gas possibly falls below the minimum value of thekinetic energy required for discharging liquid water in the interior ofthe anode gas flow passage 121 (hereinafter, referred to as “theallowable lower limit kinetic energy”).

Therefore, according to the present embodiment, the kinetic energy ofthe anode gas is ensured by adjusting the width of the pulsation to belarge when the temperature of the buffer tank 36 is high.

FIG. 6 is a view illustrating the intensity of kinetic energy of theanode gas at the same width of the pulsation upon increase of the anodepressure in accordance with the temperature of the buffer tank 36.

As illustrated in FIG. 6, it is found that the higher the temperature ofthe buffer tank 36 becomes, the lower the kinetic energy of the anodegas becomes at the same width of the pulsation.

The control of the pulsation operation according to the presentembodiment will be hereinafter described.

FIG. 7 is a flowchart explaining the control of the pulsation operationaccording to the present embodiment of the present invention. Thecontroller 4 repeatedly carries out the present routine for eachspecific time (for example, 10 ms).

The controller 4 reads the output current, a stack temperature, anambient temperature, and a vehicle speed as a stack load in Step S1.

The controller 4 computes the reference pressure of the anode pressurein the pulsation operation on the basis of the output current withreference to the table illustrated in FIG. 8 in Step S2. As illustratedin FIG. 8, the larger the output current is, the more the referencepressure of the anode pressure is increased.

The controller 4 computes the basic value of the width of the pulsation(hereinafter, referred to as “a basic width of the pulsation”) in thepulsation operation on the basis of the output current with reference tothe table illustrated in FIG. 9 in Step S3. As illustrated in FIG. 9,the larger the output current is, the more the basic width of thepulsation is increased.

The controller 4 computes the basic aperture of the purge valve on thebasis of the stack temperature with reference to the table illustratedin FIG. 10 in Step S4. As illustrated in FIG. 10, the higher the stacktemperature is, the more the basic aperture of the purge valve isincreased.

The controller 4 computes the temperature of the buffer tank 36 in StepS5. According to the present embodiment, the controller 4 computes thetemperature of the buffer tank 36 on the basis of a stack temperature,an ambient temperature, and a vehicle speed.

The controller 4 determines whether or not the temperature of the buffertank 36 is higher than a first specific temperature in step S6. Thefirst specific temperature is set at a temperature higher than thesteady temperature of the fuel cell stack 2 when the warming up thereofis completed (about 60° C.). The controller 4 carries out the processingof Step S10 if the temperature of the buffer tank 36 is higher than thefirst specific temperature and carries out the processing of Step S7 ifit is lower than the first specific temperature.

The controller 4 determines whether or not the temperature of the buffertank 36 is lower than a second specific temperature in step S7. Thesecond specific temperature is set at a temperature lower than thesteady temperature of the fuel cell stack 2 when the warming up thereofis completed. The controller 4 carries out the processing of Step S9 ifthe temperature of the buffer tank 36 is lower than the second specifictemperature and carries out the processing of Step S8 if it is higherthan the second specific temperature.

The controller 4 carries out the pulsation operation by periodicallyincreasing and decreasing the anode pressure with the central focus onthe reference pressure within the range of the basic width of thepulsation in Step S8 after determining that the temperature of thebuffer tank 36 is substantially identical to the steady temperature ofthe fuel cell stack 2.

The controller 4 determines that the temperature of the buffer tank 36is lower than the steady temperature of the fuel cell stack 2 in StepS9, carrying out the low temperature pulsation width correctionprocessing in order to make the width of the pulsation smaller than thebasic width of the pulsation. The low temperature pulsation widthcorrection processing will be later described with reference to FIG. 11.

The controller 4 carries out the high temperature pulsation widthcorrection processing in order to make the width of the pulsation largerthan the basic width of the pulsation in Step S10 after determining thatthe temperature of the buffer tank 36 is lower than the steadytemperature of the fuel cell stack 2. The high temperature pulsationwidth correction processing will be later described with reference toFIG. 13.

The controller 4 controls the aperture of the purge valve to be a basicaperture in Step S11.

FIG. 11 is a flowchart explaining the low temperature pulsation widthcorrection processing.

The controller 4 computes the maximum value of the width of thepulsation at which the lowest anode gas level in the flow passage doesnot fall below the allowable lower limit anode gas level (hereinafter,referred to as “the allowable maximum width of the pulsation”) on thebasis of the temperature of the buffer tank 36 with reference to the mapillustrated in FIG. 12 in Step S91.

FIG. 12 is a map representing a relation between the width of thepulsation and the lowest anode gas level in the flow passage uponreduction of the anode pressure for each temperature of the buffer tank36.

As illustrated in FIG. 12, if the temperature of the buffer tank 36 isthe same, the lowest anode gas level in the flow passage becomes highsince the smaller the width of the pulsation is, the shorter the timerequired for lowering the anode pressure to the lower limit pressurebecomes. In addition, if the width of the pulsation is the same, thelowest anode gas level in the flow passage becomes low since the lowerthe temperature of the buffer tank 36 is, the more the amount ofmaterial in the anode gas located in the interior of the buffer tank 36is increased and the longer the time required for lowering the anodepressure to the lower limit pressure becomes.

The controller 4 carries out the pulsation operation by periodicallyincreasing and decreasing the anode pressure within the range of theallowable minimum width of the pulsation with the central focus on thereference pressure in Step S92.

The controller 4 controls the aperture of the purge valve to be thebasic aperture in Step S93.

FIG. 13 is a flowchart explaining the high temperature pulsation widthcorrection processing.

The controller 4 computes the allowable lower limit kinetic energy onthe basis of the output current with reference to the table illustratedin FIG. 14 in Step S10. As illustrated in FIG. 14, the larger the outputcurrent is, the more the allowable lower limit kinetic energy isincreased. This is because the larger the output current is, the morewater is produced by the above-described electrode reaction (2).

The controller 4 computes the minimum value of the width of thepulsation that does not fall below the allowable lower limit kineticenergy (hereinafter, referred to as “the allowable minimum width of thepulsation”) on the basis of the temperature of the buffer tank 36 withreference to the map illustrated in FIG. 15 in Step S102.

FIG. 15 is a map representing a relation between the width of thepulsation and the kinetic energy of the anode gas upon increase of theanode pressure for each temperature of the buffer tank 36.

As illustrated in FIG. 15, if the temperature of the buffer tank 36 isthe same, the kinetic energy of the anode gas is decreased since thesmaller the width of the pulsation is, the smaller the amount of theanode gas required for increasing the anode pressure to the upper limitpressure becomes. In addition, if the width of the pulsation is thesame, the kinetic energy of the anode gas is decreased since the higherthe temperature of the buffer tank 36 is, the smaller the amount of theanode gas required for increasing the anode pressure to the upper limitpressure becomes.

The controller 4 carries out the pulsation operation by periodicallyincreasing and decreasing the anode pressure within the range of theallowable minimum width of the pulsation with the central focus on thereference pressure in Step S103.

The controller 4 controls the aperture of the purge valve to be thebasic aperture in Step S104.

FIG. 16 is a view explaining the action of the low temperature pulsationwidth correction processing according to the present embodiment. In FIG.16, a thin solid line illustrates the lowest anode gas level in the flowpassage when the temperature of the buffer tank 36 is the steadytemperature of the fuel cell stack 2 in accordance with the width of thepulsation. On the other hand, a bold line illustrates the lowest anodegas level in the flow passage when the temperature of the buffer tank 36is a specific temperature that is lower than the steady temperature ofthe fuel cell stack 2, namely, a specific temperature that is lower thanthe second specific temperature in accordance with the width of thepulsation.

As illustrated by the bold line in FIG. 16, if the pulsation operationis carried out when the temperature of the buffer tank 36 is lower thanthe steady temperature of the fuel cell stack 2, the lowest anode gaslevel in the flow passage becomes lower than the allowable lower limitanode gas level.

Therefore, the width of the pulsation in the pulsation operation iscorrected to be smaller than the basic width of the pulsation on thebasis of the temperature of the buffer tank 36 when the temperature ofthe buffer tank 36 is lower than the steady temperature of the fuel cellstack 2. Specifically, the controller 4 computes the allowable maximumwidth of the pulsation at which the lowest anode gas level in the flowpassage becomes the allowable lower limit anode gas level on the basisof the temperature of the buffer tank 36, carrying out the pulsationoperation at the computed allowable maximum width of the pulsation.Thereby, it is possible to prevent the lowest anode gas level in theflow passage from falling below the allowable lower limit anode gaslevel upon reduction of the anode pressure.

FIG. 17 is a view explaining the action of the high temperaturepulsation width correction processing according to the presentembodiment. In FIG. 17, a thin solid line illustrates the kinetic energyof the anode gas when the temperature of the buffer tank 36 is thesteady temperature of the fuel cell stack 2 in accordance with the widthof the pulsation. On the other hand, a bold line illustrates the kineticenergy of the anode gas when the temperature of the buffer tank 36 is aspecific temperature that is higher than the steady temperature of thefuel cell stack 2, namely, a specific temperature that is higher thanthe first specific temperature, in accordance with the width of thepulsation.

As illustrated by the bold line in FIG. 17, if the pulsation operationis carried out when the temperature of the buffer tank 36 is higher thanthe steady temperature of the fuel cell stack 2, the kinetic energy ofthe anode gas becomes smaller than the allowable lower limit kineticenergy.

Therefore, the width of the pulsation in the pulsation operation iscorrected to be larger than the basic width of the pulsation on thebasis of the temperature of the buffer tank 36 when the temperature ofthe buffer tank 36 is higher than the steady temperature of the fuelcell stack 2. Specifically, the controller 4 computes the allowableminimum width of the pulsation at which the kinetic energy of the anodegas becomes the allowable lower limit kinetic energy on the basis of thetemperature of the buffer tank 36, carrying out the pulsation operationat the computed allowable minimum width of the pulsation. Thereby, it ispossible to prevent the kinetic energy of the anode gas from fallingbelow the allowable lower limit kinetic energy upon increase of theanode pressure.

According to the above-described present embodiment, the width of thepulsation in the case of carrying out the pulsation operation iscorrected on the basis of the temperature of the buffer tank 36.

Specifically, when the temperature of the buffer tank 36 is lower thanthe steady temperature of the fuel cell stack 2, namely, lower than thesecond specific temperature, the width of the pulsation in the pulsationoperation is made smaller than the width of the pulsation that is setwhen the temperature of the buffer tank 36 is substantially identical tothe steady temperature of the fuel cell stack 2.

Thereby, it is possible to prevent the lowest anode gas level in theflow passage from falling below the allowable lower limit anode gaslevel upon reduction of the anode pressure since the time until theanode pressure falls below the lower limit pressure becomes short.Accordingly, there is no possibility that the voltage is turned to anegative voltage due to inhibition of the above-descried electrodereactions (1) and (2), and this makes it possible to carry out stableelectric power generation and to prevent deterioration of the fuel cell10.

In addition, when the temperature of the buffer tank 36 is higher thanthe steady temperature of the fuel cell stack 2, namely, higher than thefirst specific temperature, the width of the pulsation in the pulsationoperation is made larger than the width of the pulsation that is setwhen the temperature of the buffer tank 36 is substantially identical tothe steady temperature of the fuel cell stack 2.

Thereby, it is possible to prevent the kinetic energy of the anode gasfrom falling below the allowable lower limit kinetic energy since thekinetic energy of the anode gas is increased for the increased amount ofthe width of the pulsation. Therefore, the discharge performance ofliquid water can be ensured, making it possible to prevent flooding frombeing generated in the anode gas flow passage 121.

Second Embodiment

Subsequently, the second embodiment of the present invention will bedescribed. The present embodiment is different from the first embodimentin that the aperture of the purge valve 38 is corrected in accordancewith the moisture status of the electrolyte membrane 111. Hereinafter,the different point will be mainly described. In the followingembodiment, the parts with the same functions as the above-describedfirst embodiment are provided with the same reference codes and thedescription thereof is herein omitted.

In the first embodiment, when the temperature of the buffer tank 36 islower than the steady temperature of the fuel cell stack 2, the width ofthe pulsation in the pulsation operation is smaller than the width ofthe pulsation that is set when the temperature of the buffer tank 36 issubstantially identical to the steady temperature of the fuel cell stack2.

However, if the width of the pulsation in the pulsation operation ismade small, the kinetic energy of the anode gas upon increase of theanode pressure is decreased, then the discharge performance of liquidwater in the interior of the anode gas flow passage 121 is deteriorated.Accordingly, when the electrolyte membrane 111 is in a wet state with ahigh water content, it is preferable that the reduced width from thebasic width of the pulsation is made small as much as possible.

FIG. 18 is a view illustrating a relation between a width of thepulsation and the lowest anode gas level in the flow passage when thetemperature of the buffer tank 36 is a specific temperature inaccordance with the aperture of a purge valve 38.

As illustrated in FIG. 18, the higher the anode gas level in theinterior of the buffer tank 36 is made by increasing the aperture of thepurge valve 38, the higher the lowest anode gas level in the flowpassage with the same width of the pulsation upon reduction of the anodepressure is made. This is because the higher the anode gas level in theinterior of the buffer tank 36 is made, the more nitrogen in theanode-off gas to flow backward from the buffer tank 36 side into theanode gas flow passage 121 is reduced, then nitrogen stored in thevicinity of the stagnation point is also reduced.

Accordingly, as illustrated in FIG. 18, the larger the aperture of thepurge valve 38 is made, the larger the allowable maximum width of thepulsation is made.

Therefore, according to the present embodiment, when the electrolytemembrane 111 is in a moisture status, the anode gas level in theinterior of the purge valve 38 is made higher than usual by increasingthe aperture of the purge valve 38. Thereby, it is possible to increasethe allowable maximum width of the pulsation, making it possible toreduce the reduced width in the width of the pulsation from the basicwidth of the pulsation.

FIG. 19 is a flowchart explaining the low temperature pulsation widthcorrection processing according to the present embodiment.

The controller 4 computes an inner high frequency resistance (HFR: HighFrequency Resistance) (hereinafter, referred to as “an innerresistance”) of the fuel cell stack 2 in order to determine the moisturestatus of the electrolyte membrane 111 in Step S291. It has been knownthat there is a correlation between the moisture status of theelectrolyte membrane 111 and the inner resistance of the fuel cell stack2, in which correlation, the lower the inner resistance of the fuel cellstack 2 is, the more the moisture in the membrane exists, then theelectrolyte membrane 111 is in the moisture status.

The controller 4 determines whether or not the inner resistance of thefuel cell stack 2 is smaller than a specific value in Step S292. Thecontroller 4 determines that the electrolyte membrane 111 is in themoisture status if the inner resistance of the fuel cell stack 2 issmaller than the specific value, carrying out the processing of StepS293. On the other hand, if the inner resistance of the fuel cell stack2 is not less than the specific value, the controller 4 carries out theprocessing of Step S92.

The controller 4 computes the correction amount of the aperture of thepurge valve 38 on the basis of the inner resistance of the fuel cellstack 2 with reference to the table illustrated in FIG. 20 in Step S293.As illustrated in FIG. 20, the smaller the inner resistance of the fuelcell stack 2 is, namely, the more the moisture in the membrane of theelectrolyte membrane 111 exists, the aperture of the purge valve 38 isincreased by increasing the correction amount of the aperture of thepurge valve 38.

The controller 4 computes the correction amount of the allowable maximumwidth of the pulsation on the basis of the correction amount of theaperture of the purge valve 38 with reference to the table illustratedin FIG. 21 in Step S294. As illustrated in FIG. 21, the larger thecorrection amount of the aperture of the purge valve 38 is, the largerthe correction amount of the allowable maximum width of the pulsationbecomes.

The controller 4 carries out the pulsation operation at the width of thepulsation in which the correction amount is added to the allowablemaximum width of the pulsation (hereinafter, referred to as “a correctedallowable maximum width of the pulsation”) with the central focus on thereference pressure in Step S295.

The controller 4 controls the aperture of the purge valve 38 to be thecorrected aperture in which the correction amount of the aperture isadded to the basic aperture in Step S296.

FIG. 22 is a view explaining the action of the low temperature pulsationwidth correction processing according to the present embodiment. In FIG.22, a thin solid line illustrates the lowest anode gas level in the flowpassage when the temperature of the buffer tank is a specifictemperature that is lower than the steady temperature of the fuel cellstack and the aperture of the purge valve is a basic aperture inaccordance with the width of the pulsation. On the other hand, a boldline illustrates the lowest anode gas level in the flow passage when thetemperature of the buffer tank is a specific temperature that is lowerthan the steady temperature of the fuel cell stack and the aperture ofthe purge valve is larger than the basic aperture in accordance with thewidth of the pulsation.

As represented by a bold line in FIG. 22, when the temperature of thebuffer tank is lower than the steady temperature of the fuel cell stackand the electrolyte membrane 111 is in the moisture status, the apertureof the purge valve 38 is made larger than the basic aperture inaccordance with the inner resistance of the fuel cell stack 2. Thereby,it is possible to increase the allowable maximum width of the pulsationto the corrected allowable maximum width of the pulsation since thelowest anode gas level in the flow passage can be made higher, making itpossible to decrease the reduced width in the width of the pulsationfrom the basic width of the pulsation.

As a result, according to the present embodiment, the same advantages asthe first embodiment can be obtained and further, the dischargeperformance of liquid water when the electrolyte membrane 111 is in themoisture status can be improved and this makes it possible to furtherprevent flooding from being generated in the anode gas flow passage 121.

Third Embodiment

Subsequently, the third embodiment of the present invention will bedescribed. The present embodiment is different from the first embodimentin that the width of the pulsation is made larger and the aperture ofthe purge valve 38 is corrected upon carrying out the high temperaturepulsation width correction processing. Hereinafter, the different pointwill be mainly described.

FIG. 23 is a view illustrating a relation between the width of thepulsation, the kinetic energy of the anode gas, and the lowest anode gaslevel in the flow passage when the temperature of the buffer tank 36 isa specific temperature that is higher than the steady temperature of thefuel cell stack 2.

As illustrated in FIG. 23, according to the first embodiment, when thetemperature of the buffer tank 36 is higher than the steady temperatureof the fuel cell stack 2, the pulsation operation is carried out at theallowable minimum width of the pulsation by correcting the width of thepulsation in the pulsation operation to be larger than the basic widthof the pulsation in order to prevent the kinetic energy of the anode gasdoes from falling below the allowable lower limit kinetic energy.

However, the allowable lower limit kinetic energy is varied inaccordance with the operation status of the fuel cell system. Therefore,as illustrated in FIG. 23, in the case that the allowable lower limitkinetic energy is comparatively small, the lowest anode gas level in theflow passage when the pulsation operation is carried out at theallowable minimum width of the pulsation is sometimes higher than theallowable lower limit anode gas level.

On the other hand, in the case that the allowable lower limit kineticenergy is comparatively large, the lowest anode gas level in the flowpassage when the pulsation operation is carried out at the allowableminimum width of the pulsation is sometimes lower than the allowablelower limit anode gas level.

Therefore, according to the present embodiment, when the controller 4determines that the lowest anode gas level in the flow passage duringthe pulsation operation at the allowable minimum width of the pulsationis larger than the allowable lower limit anode gas level, the apertureof the purge valve 38 is made smaller than the basic aperture in orderto decrease the lowest anode gas level in the flow passage to theallowable lower limit anode gas level. Thereby, it is possible to reducethe anode gas amount to be discharged from the purge passage, making itpossible to improve a fuel efficiency.

On the other hand, when the controller 4 determines that the lowestanode gas level in the flow passage during the pulsation operation atthe allowable minimum width of the pulsation is lower than the allowablelower limit anode gas level, the aperture of the purge valve 38 is madelarger than the basic aperture in order to increase the lowest anode gaslevel in the flow passage to the allowable lower limit anode gas level.Hereinafter, the high temperature pulsation width correction processingaccording to the present embodiment will be described.

FIG. 24 is a flowchart explaining the high temperature pulsation widthcorrection processing according to the present embodiment.

The controller 4 computes the lowest anode gas level in the flow passageduring the pulsation operation at the allowable minimum width of thepulsation with reference to the above-described map of FIG. 12 in StepS301.

The controller 4 determines whether or not the computed lowest anode gaslevel in the flow passage is not less than the allowable lower limitanode gas level in Step S302. The controller 4 carries out theprocessing of Step S303 if the computed lowest anode gas level in theflow passage is not less than the allowable lower limit anode gas level.On the other hand, if the computed lowest anode gas level in the flowpassage is lower than the allowable lower limit anode gas level, theprocessing of Step S306 is carried out.

The controller 4 computes the correction amount of the aperture of thepurge valve 38 on the basis of the difference in level obtained bysubtracting the allowable lower limit anode gas level from the computedlowest anode gas level in the flow passage with reference to the tableillustrated in FIG. 25 in Step S303. As illustrated in FIG. 25, thecorrection amount of the aperture of the purge valve 38 is set such thatthe larger the level difference becomes, the smaller the aperture of thepurge valve 38 becomes compared to the basic aperture.

The controller 4 makes the aperture of the purge valve 38 to be thecorrected aperture in which the correction amount of the aperture isadded to the basic aperture of the purge valve 38 in Step S304.

The controller 4 carries out the pulsation operation at the allowablemaximum width of the pulsation with the central focus on the referencepressure in Step S305.

The controller 4 computes the correction amount of the aperture of thepurge valve 38 on the basis of the difference in level obtained bysubtracting the allowable lower limit anode gas level from the computedlowest anode gas level in the flow passage with reference to the tableillustrated in FIG. 26 in Step S306. As illustrated in FIG. 26, thecorrection amount of the aperture of the purge valve 38 is set such thatthe larger the level difference becomes, the larger the aperture of thepurge valve 38 becomes compared to the basic aperture.

Subsequently, the action of the high temperature pulsation widthcorrection processing according to the present embodiment will bedescribed with reference to FIG. 27 and FIG. 28.

FIG. 27 is a view explaining the action in the case that the lowestanode gas level in the flow passage during the pulsation operation atthe allowable minimum width of the pulsation is higher than theallowable lower limit anode gas level.

As illustrated in FIG. 27, in the case that the pulsation operation iscarried out at the allowable minimum width of the pulsation, when thelowest anode gas level in the flow passage is higher than the allowablelower limit anode gas level, the aperture of the purge valve 38 is madesmaller than the basic aperture in order to decrease the lowest anodegas level in the flow passage to the allowable lower limit anode gaslevel. Thereby, it is possible to reduce the anode gas amount to bedischarged from the purge passage, making it possible to improve a fuelefficiency.

FIG. 28 is a view explaining the action in the case that the lowestanode gas level in the flow passage during the pulsation operation atthe allowable minimum width of the pulsation is the allowable lowerlimit anode gas level.

As illustrated in FIG. 28, in the case that the pulsation operation iscarried out at the allowable minimum width of the pulsation, when thelowest anode gas level in the flow passage falls below the allowablelower limit anode gas level, the aperture of the purge valve 38 is madelarger than the basic aperture in order to increase the lowest anode gaslevel in the flow passage to the allowable lower limit anode gas level.Thereby, it is possible to carry out more stable electric generationsince the lowest anode gas level in the flow passage can be preventedfrom falling below the allowable lower limit anode gas level even if thetemperature of the buffer tank 36 is higher than the steady temperatureof the fuel cell stack 2.

According to the above-described present embodiment, when thetemperature of the buffer tank 36 is higher than the steady temperatureof the fuel cell stack 2, the width of the pulsation in the pulsationoperation is corrected to be larger than the basic width of thepulsation in order to prevent the kinetic energy of the anode gas fromfalling below the allowable lower limit kinetic energy, carrying out thepulsation operation at the allowable minimum width of the pulsation.

Then, when the lowest anode gas level in the flow passage during thepulsation operation at the allowable minimum width of the pulsation ishigher than the allowable lower limit anode gas level, the aperture ofthe purge valve 38 is made smaller than the basic aperture in order todecrease the lowest anode gas level in the flow passage to the allowablelower limit anode gas level. Thereby, it is possible to reduce the anodegas amount to be discharged from the purge passage, making it possibleto improve a fuel efficiency.

On the other hand, when the lowest anode gas level in the flow passageduring the pulsation operation at the allowable minimum width of thepulsation is lower than the allowable lower limit anode gas level, theaperture of the purge valve 38 is made larger than the basic aperture inorder to increase the lowest anode gas level in the flow passage to theallowable lower limit anode gas level. Thereby, it is possible to carryout more stable electric generation even if the temperature of thebuffer tank 36 is higher than the steady temperature of the fuel cellstack 2 since the lowest anode gas level in the flow passage can beprevented from falling below the allowable lower limit anode gas level.

Fourth Embodiment

Subsequently, the fourth embodiment of the present invention will bedescribed. The present embodiment is different from the first embodimentin that the aperture of purge valve 38 is only corrected in order toprevent the lowest anode gas level in the flow passage from fallingbelow the allowable lower limit anode gas level when the temperature ofthe buffer tank 36 is lower than the steady temperature of the fuel cellstack 2. Hereinafter, the different point will be mainly described.

FIG. 29 is a flowchart explaining the control of the pulsation operationaccording to the present embodiment. The controller 4 repeatedly carriesout the present routine for each specific time (for example, 10 ms).

The controller 4 carries out the low temperature pulsation widthcorrection processing in Step S49.

FIG. 30 is a flowchart explaining the low temperature correctionprocessing of the aperture of the purge valve.

The controller 4 computes the lowest anode gas level in the flow passageduring the pulsation operation at the basic width of the pulsation withreference to the above-described map of FIG. 12 in Step S491.

The controller 4 computes the correction amount of the aperture of thepurge valve 38 on the basis of the difference in level obtained bysubtracting the allowable lower limit anode gas level from the computedlowest anode gas level in the flow passage with reference to the tableillustrated in FIG. 31 in Step S492. As illustrated in FIG. 31, thecorrection amount of the aperture of the purge valve 38 is computed suchthat the larger the level difference becomes, the larger the aperture ofthe purge valve 38 becomes compared to the basic aperture.

The controller 4 makes the aperture of the purge valve 38 to be thecorrected aperture in which the correction amount of the aperture isadded to the basic aperture of the purge valve 38 in Step S493.

The controller 4 carries out the pulsation operation at the basic widthof the pulsation with the central focus on the reference pressure inStep S494.

FIG. 32 is a view explaining the action of the low temperaturecorrection processing of the aperture of the purge valve according tothe present embodiment, namely, a view explaining a relation between thewidth of the pulsation and the lowest anode gas level in the flowpassage when the temperature of the buffer tank 36 is a specifictemperature lower than the steady temperature of the fuel cell stack 2.

As illustrated in FIG. 32, if the pulsation operation is carried out atthe basic width of the pulsation when the temperature of the buffer tank36 is lower than the steady temperature of the fuel cell stack 2, thelowest anode gas level in the flow passage becomes lower than theallowable lower limit anode gas level.

Therefore, according to the present embodiment, the aperture of thepurge valve 38 is made larger than the basic aperture in order toincrease the lowest anode gas level in the flow passage to the allowablelower limit anode gas level. Thus, it is also possible to obtain thesame advantage as the first embodiment by only correcting the apertureof the purge valve 38 without correction of the width of the pulsation.

The embodiments of the present invention have been described above;however, the above-described embodiments merely indicate some examplesof the application of the present invention with no intention to limitthe technical scope of the present invention to the specificconfigurations of the above-described embodiments.

According to the above-described respective embodiments, the temperatureof the buffer tank 36 is computed by the operation; however, the presentinvention is not limited to this. For example, the temperature of thebuffer tank 36 may be directly detected by providing a temperaturesensor to the buffer tank 36.

In addition, according to the above-described respective embodiments,the width of the pulsation is corrected on the basis of the temperatureof the buffer tank 36; however, the present invention is not limited tothis. For example, using the volume of the anode gas supply passage 32from the pressure-adjusting valve 33 to the fuel cell stack 2(hereinafter, referred to as “an upstream buffer volume”) to resemblethe buffer tank, the width of the pulsation may be corrected on thebasis of the temperature of this upper buffer volume as is the case withthe above-described respective embodiments. In addition, the width ofthe pulsation may be corrected in accordance with the difference intemperature between the fuel cell stack 2 and the buffer tank 36.

Further, according to the first embodiment, the pulsation operation maybe carried out at the allowable maximum width of the pulsation in orderto prevent the lowest anode gas level in the flow passage from fallingbelow the allowable lower limit anode gas level by correcting the widthof the pulsation to be smaller than the basic width of the pulsationwhen the temperature of the buffer tank 36 is lower than that of thefuel cell stack 2; however, the present invention is not limited tothis. For example, the pulsation operation may be carried out whiledecreasing the width of the pulsation less than the allowable maximumwidth of the pulsation since the smaller the width of the pulsation is,the higher the anode gas level in the flow passage becomes.

In this case, it is possible to decrease the width of the pulsation lessthan the allowable maximum width of the pulsation within the range thatthe kinetic energy of the anode gas does not fall below the allowablelower limit kinetic energy, namely, the range that the width of thepulsation does not fall below the allowable minimum width of thepulsation since the smaller the width of the pulsation is made, thelower the kinetic energy of the anode gas becomes. Thereby, it ispossible to ensure the discharge performance of liquid water whileensuring the electric power generation performance.

The present application claims the priority based on Japanese PatentApplication No. 2011-124220 filed with Japan Patent Office on Jun. 2,2011 and all contents of this application are incorporated in thepresent specification by reference.

The invention claimed is:
 1. A fuel cell system that generates electricpower by supplying anode gas and cathode gas to a fuel cell, comprising:a control valve adapted to control a pressure of the anode gas to besupplied to the fuel cell; a buffer unit adapted to store an anode-offgas to be discharged from the fuel cell; and a controller programmed to:control the control valve in order to periodically increase and decreasethe pressure of the anode gas at a specific width of a pulsation; andcorrect the width of the pulsation based on a temperature of an upstreambuffer volume comprising an anode gas flow passage from the controlvalve to the fuel cell.